1. Field of the Invention
This invention relates to a device for performing plasma chemical vapor deposition (PCVD) for producing coated glass tubes for the drawing of optical fibers. This invention further relates to a device that uses a microwave applicator, and the applicator itself with a profile that allows for a uniform coating across a greater length of the glass tube.
2. Technology Review
The transmission of communications by means of optical fibers is commercially important today. Optical fibers have acquired an increasingly important role in the field of communications, frequently replacing existing copper wires. This trend has significantly impacted local area networks (i.e., fiber-to-home uses), which has seen a vast increase in the usage of optical fibers. Further increases in the use of optical fibers in local loop telephone and cable TV service are expected, as local fiber networks are established to deliver even greater volumes of information in the form of data, audio and video signals to residential and commercial users.
This form of transmission is done by sending a beam of light through an optically clear fiber. Optical fibers typically contain a glass core, a glass cladding, and at least two polymer coatings, i.e., a primary (or inner) coating and a secondary (or outer) coating. The glass core and cladding of the optical fiber are formed by a number of methods and devices. One of those methods starts by depositing a PCVD coating on the inside of a glass tube by generating a plasma on the inside of a glass tube. The PCVD method of coating a glass tube is distinguished from other methods by its ability to deposit fine layers of glass with high dopant concentrations thereby creating radial optical index profiles with fine radial structure and large index variations. The plasma is created when the gas mixture in the tube is partially ionized by alternating RF or microwave fields. This plasma then dissociates and couples thermal energy into the neutral gas mixture initiating chemical processes which lead eventually to the deposition of doped or undoped glass on the interior of the tube. A schematic of a conventional applicator is shown in FIG. 1. As shown in FIG. 1, substrate tube 1 is positioned within oven 2 and the plasma is traversed along the tube 1 by moving a microwave applicator 9 mechanically within the oven relative to the longitudinal axis of the tube between the two inside ends of the oven (the length between the two inside ends of the oven is herein known as Loven). The same single applicator 9 is shown in FIG. 1 at the left most end of a traverse (9A) and at the right most end of a traverse (9B). Between these two positions is the usable product length deposition zone A, as will be discussed below. The microwave applicator, having a maximum length parallel to the longitudinal axis of the tube (herein known as Lapplicator) is mechanically reversed just before the applicator reaches the inside wall of the oven and makes another pass over the tube to create another layer. The microwave applicator continues to make passes until the desired coating thickness and profile is built up. The coated tube is called a hollow preform. The number of layers in the coating and the composition of these layers influences the desired refractive index profile, which is to be employed in the resultant optical fiber. After a glass coating of the desired radial composition profile and of the desired amount has been deposited, the tube (or hollow preform) is collapsed to form a collapsed preform. The collapsed preform can then be drawn directly into optical fiber, or can be overclad then drawn into an optical fiber.
In depositing a coating on an optical fiber preform by PCVD, one of the drawbacks is in the axial uniformity of deposited glass layers along the deposition zone. Deviations in the axial uniformity of the layers along the tube axis occur at the starting and ending regions of the tube near the positions where the applicator direction is reversed. Layer and ultimately coating thickness tapers in these regions. These unusable coating areas are generally at both ends of the tube and are at least the length of the applicator (Lapplicator) at each end of the tube. Therefore, the usable deposition zone (Ldeposition) (that area of the preform where the thickness of the coating layers is substantially the same and where there is no geometric taper) of coated tubes made with conventional coating devices is equal to or less than Lovenxe2x88x922(Lapplicator). The non-axially uniform regions must be removed prior to drawing the optical fiber to prevent quality problems with the fiber, resulting in increased manufacturing expenses as a result of lower yields. This further results in an optical preform that is significantly shorter than that which is theoretically possible and therefore decreases the amount of continuous length of fiber that the manufacturers can produce.
The present invention is directed to device and a microwave applicator used in the device for producing optical fiber preforms, which may be drawn into optical fibers.
In one embodiment of the present invention, a device for depositing a coating on the inside of a substrate tube is provided which comprises an oven having two end walls for heating the substrate tube, the end walls each having inner and outer surfaces and an opening between the inner and outer surfaces through which the substrate tube can be mounted; and an applicator located in the oven and positioned to be circumferential about a longitudinal axis of the substrate tube for generating a plasma zone from a mixture of gases in the tube by application of energy. The applicator comprises a body and at least one extension positioned about the longitudinal axis of the substrate tube and extending from the body, wherein both the body and the extensions are used to direct energy into the tube and at least one of the extensions has a profile capable of moving at least partially into the opening in the end wall of the oven.
In another embodiment of the present invention, a method of coating the inside of substrate tube is provided which comprises the steps of heating a substrate tube with an oven for heating the substrate tube, the oven comprising two end walls having inner and outer surfaces and an opening between the inner and outer surfaces of each of the end walls through which the substrate tube is mounted; passing a gas mixture through the substrate tube at low pressures; applying energy to react the gas mixture to form a plasma in the substrate tube with an applicator within the furnace, wherein the applicator comprises a body and at least one extension which is substantially symmetric about the substrate tube extending from the body, wherein both the body and the extensions direct the energy into the substrate tube; and moving the applicator along the longitudinal axis of the substrate tube wherein the extension of the applicator has a profile which allows it to move at least partially into the opening in the end wall of the oven.
In still another embodiment of the present invention, a microwave applicator capable of use in an oven at temperatures greater than 1000xc2x0 C. is provided which comprises a body and at least one extension which is substantially symmetric about a longitudinal axis through the body and the at least one extension, extending from the body, wherein both the body and the extensions are used to direct microwaves into the tube and the at least one extension is capable of withstanding the oven temperatures without being insulated and/or water cooled and without substantially distorting or bending.
In still another embodiment of the present invention, a device for depositing a coating on the inside of a substrate tube is provided which comprises an oven having two end walls having inner and outer surfaces and having a distance between the inner surfaces of the two end walls known as Loven; and a microwave applicator comprising a body and at least one extension extending from the body, located in the furnace and circumferentially about the substrate tube mounted through and between the two end walls for generating a plasma in the tube from a mixture of gases by application of microwaves, the microwave applicator having a maximum length along or parallel to the longitudinal axis of the substrate tube known as Lapplicator; wherein the device is capable of depositing a usable film in the tube across a coating deposition zone wherein the length of the usable deposition zone is Ldeposition zone, and Ldeposition zone greater than Lovenxe2x88x922(Lapplicator). The device and the applicator therefore produce a tube that has a longer usable coated deposition zone.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.